Adults possess a rich set of conceptions about the internal cognitions and mental states of themselves and others. These conceptions constitute a naive theory of mind and as such are crucial to a mature understanding of self and others. But what, if anything, do young children know about the mind? During the current grant period I have elaborated a theory of children's developing understanding of the mind. This theory sketches the nature of our everyday adult theory of mind, and proposes several phases in the development of this basic understanding. Consideration of this domain of understanding and of this developmental theory raises several questions. One concerns how children's understanding of persons fits with, yet differentiates from, their understanding of physical objects or of plants and animals--how naive psychology developmentally relates to naive physics and naive biology. A second concerns how understandings of different mental states--e.g., beliefs versus desires versus emotions versus percepts--relate to one another developmentally in the child's larger understanding of persons. A third concerns issues of ecological validity and conceptual importance--whether a mentalistic understanding of persons is indeed common and basic to person conception and social understanding or rather rare and elicited only in contrived laboratory tasks? A fourth concerns the nature and timing of developmental transitions in this domain. Relatedly, a fifth question concerns clarifying the research findings on this topic. As relevant research has quickly accumulated over the last few years it has become increasingly inconsistent--an initial set of findings replicated across several different laboratories, countries,m and tasks have yielded to an array of, at times, contradictory findings and quite divergent theoretical interpretations. I propose a variety of interrelated studies to address these five questions: research relating children's psychological understanding to their physical and biological reasoning; research encompassing and relating children's understanding of persons' thoughts, desires, emotions, and perceptions; research coupling laboratory methods with more observational natural language findings; a meta-analysis to clarify the nature of extant findings across many individual studies; and several multi-measure investigations designed to more firmly assess developmental patterns, sequences, and correlates of children's understanding of mind.